Generally, a seat lifter apparatus for a vehicle for moving a seat for the vehicle (a driven-side member) in an upper-lower direction of the vehicle by rotating a motor (a driving-side member) includes a brake device in order to restrict a reverse rotation of the motor at the time when an excessive load is applied on the seat for the vehicle. For example, JPS59-117922A discloses a coupler device, serving as the brake device, in which a characteristic of a coil spring is utilized. More specifically, the coupler device includes a driving member, a driven member, a small-diameter coil spring and a large-diameter coil spring, which are accommodated within a case. When the driving member is rotated, a diameter of the small-diameter coil spring is expanded and the small-diameter coil spring is spaced away from an inner wall of the case while a diameter of the large-diameter coil spring is reduced and the large-diameter coil sprig is spaced away from an outer wall of the case. Therefore, a slide resistance is not generated between the case and each of the small-diameter and large-diameter coil springs, and the driven member is rotated in accordance with the rotation of the driving member. On the other hand, when the driven member is rotated, one of the small-diameter and large-diameter coil springs contacts one of the inner and outer walls of the case. Therefore, a slide resistance is generated between the case and one of the small-diameter and large-diameter coil springs, and the driving member is not rotated in accordance with the rotation of the driven member. Accordingly, even when a large load is applied on the seat for the vehicle, a reverse rotation of the motor is restricted and the motor is maintained to be in a state where a braking force is applied.
Further, generally, rails of a seat slide apparatus for a vehicle for moving a seat for the vehicle in a front-rear direction of the vehicle is fixed to a floor of the vehicle in an inclined manner so that front portions of the rails are arranged to be higher than rear portions thereof. The seat for the vehicle is moved toward a front side of the vehicle when a physically relatively small occupant, such as a child and the like, is seated thereon. Therefore, the rails are arranged in the above-mentioned inclining manner in order to raise eyes' position of the occupant and to ensure a sufficient filed of view. However, according the seat slide apparatus for the vehicle for moving the seat for the vehicle in the front-rear direction of the vehicle by means of a motor, the moving speed of the seat for the vehicle may be reduced when the seat for the vehicle is moving forward, while the moving speed of the seat for the vehicle may be increased when the seat for the vehicle is moving rearward, due to the inclination of the rails, depending on a weight of the occupant. The moving speed of the seat for the vehicle may be increased in an accelerated manner when the seat for the vehicle is moving rearward so as to move downward on the inclination, compared to when the seat for the vehicle is moving forward so as to move up on the inclination, thereby accelerating the rotation of the motor.
An increase of a rotational speed of the motor may cause an increase of an operational noise of the motor, frictional noise of the gear shafts and the like, vibration of engaged gears and the like. Generally, an operational noise becomes larger when the seat for the vehicle is moving rearward than when the seat for the vehicle is moving forward, thereby causing to generate a difference of noise. Accordingly, a difference of an operational characteristic is generated between the forward and rearward movements of the seat for the vehicle. In a case where the conventional coupler device of JPS59-117922A is applied to the seat slide apparatus for the vehicle, the coupler device may function as the brake device for restricting the reverse rotation of the motor by applying a braking force to the motor when an excessive load is applied on the seat for the vehicle. However, the difference of the moving speed between the forward and rearward movements of the seat for the vehicle may not be removed and the difference of the operational noise between the forward and rearward movements of the seat for the vehicle may not be reduced. Accordingly, the difference of the operational characteristic between the forward and rearward movements of the seat for the vehicle may remain.
A need thus exists for a seat slide apparatus for a vehicle, which is not susceptible to the drawback mentioned above.